The invention relates to a method for ascertaining an insulation resistance of a test object that is to be examined, wherein the test object, in particular, may be a high-voltage system or a component of a high-voltage system.
The invention furthermore relates to a device for ascertaining an insulation resistance of a test object that is to be examined, wherein the device has a circuit arrangement, a measuring unit and an evaluation unit.
In addition, the invention relates to a high-voltage battery system having a device for ascertaining an insulation resistance of the high-voltage battery system.
Unwanted electrical potential differences representing a potential danger can form in a high-voltage system due to insulation faults or due to a change in the insulation resistance, for example due to material ageing or material damage. On the one hand, the high-voltage system may be damaged, and, on the other hand, the danger of electric shocks exists for persons using or working on the high-voltage system.
A device is known from document EP 2 570 281 A1 for measuring the insulation resistance of a high-voltage battery system. In order to measure the insulation resistance, a reference resistance is provided which can be connected via switching elements in parallel with the insulation resistances that are to be ascertained. The high-voltage battery itself is used as a voltage source for ascertaining the insulation resistance.
In addition, a method is known from document CN 102 944 750 for measuring the insulation resistance of a device during the operation of the device.
In particular, the connection of a voltage divider consisting of high-impedance resistors is known for ascertaining the insulation resistance in high-voltage systems. In IT systems (IT: Isolé Terre [not connected to ground]), both potentials HV+ and HB− must be isolated from ground.
Resistive measurements used to determine insulation resistance have the disadvantage that the insulation resistance itself is negatively influenced by the resistive measurement. If a measurement of the insulation resistance is carried out by means of resistive measurements, an electrical connection is disadvantageously established via the resistors required for this purpose. In addition, relays are normally required for insulation resistance measurements of this type, such as, for example, in the document EP 2 570 289 A1, said relays being subject to wear.
Against this background, an object of the invention is to improve the measurement of the insulation resistance of a test object that is to be examined in such a way that the potentials HV+ and HV− remain electrically isolated from the vehicle ground when the insulation resistance is ascertained. In addition, a facility for ascertaining the insulation resistance which results in less wear is advantageously to be provided.